Numerous machines have been devised for producing flat lapped surfaces on machined, ground, or precision cast work pieces. A common example of such a lapping machine comprises a lap wheel or plate, usually annular in configuration, which is centrally rotated about a vertical axis and against which the work pieces in confined positions are placed. The movement of the lap wheel relative to the work pieces thus brings about abrasive wear of the surface of each work piece that engages the contacting surface of the lap wheel. A prime consideration in a machine of this class is the contacting surface itself on the lap wheel should be planar or near planar. Abrasive engagement of such contacting surface with the work pieces can wear it unevenly, and when such uneven wear occurs, the effectiveness of lapping is reduced.
One effort to maintain the contacting surface on the lap wheel planar includes the provision of a truing and retaining ring rested on the contacting surface annularly of one or more of the work pieces and of the pressure plate of the machine. The ring serves the dual purpose of holding one or more of the work pieces within itself, and further in rotating about its own central axis against the contacting surface of the lap wheel tends to wear the contacting surface in a more uniform pattern as compared to when the specific work pieces only engage the contacting surface. Efforts in this regard include U.S. Pat. No. 3,110,988 which pertains to the means for selectively rotating these truing and retaining rings under power in order to more evenly wear the contacting surface of the lap wheel.
Of prime concern also is the manner in which the pressure plate of the lapping machine engages the upper side of the work pieces, or a fixture holding the work pieces, and biases the work pieces toward the lap wheel. The pressure plate itself is normally mounted to rotate about a vertical axis laterally offset from the vertical axis of rotation of the lap wheel, and means are provided for axially biasing the pressure plate in the direction towards the lap wheel to create adjustable pressures of contact between the lap wheel and the work pieces. Bearing structure generally is provided between each pressure plate and its mounting shaft so as to allow the pressure plate to rotate freely about the shaft.
Notwithstanding the specific structural means employed to maintain the contacting surface of the lap wheel planar, the surface does become out of plane and consequently a cant pressure can build up between the pressure plate and the supporting shaft to the extent that the pressure plate must wobble or rotate about an axis that is not coincidental to the axis of the shaft. This canting pressure can be caused by the dished shape assumed by the rotating lap wheel, either concave or convex toward the work pieces. This canting pressure further can be caused by a slight lateral deflection of the pressure plate shaft itself, or by wobble of the shaft which is generally but an extension of the pressure cylinder rod. This canting pressure can even yet be caused to varying degrees because of differential drag incurred between the lap wheel and the work pieces confined beneath the pressure plate as the lapping operation progresses. Because of the intended continuing movement or rotation of the pressure plate and of the truing and retaining ring about each respective axis, this canting misalignment would occur during each rotation of the pressure plate.
A further consideration in the shaft mounting of the pressure plate is the magnitude of the axial force encountered between these components, and the resulting lateral forces encountered caused by the frictional drag of the work piece against the lap wheel. The axial forces are adjusted as dictated by many factors including the size or durability of the work pieces and the speed or stage of cut of the lap wheel, but can range between several hundred and several thousand pounds; while the lateral forces are only a percentage of the axial forces. The mounting structure between the pressure plate and its supporting shaft must be capable of accommodating these large forces while yet allowing the pressure plate to rotate and swivel freely.